Impurity-induced sub-gap bound gap states in alkali doped iron chalcogenide superconductors

TL;DR

This study investigates how impurities affect the local density of states in alkali doped iron chalcogenide superconductors, revealing impurity-induced bound states that depend on the superconducting pairing symmetry and doping level.

Contribution

It provides a detailed analysis of impurity effects on sub-gap states in d-wave and s-wave superconductors, highlighting the role of impurity type and doping.

Findings

Sub-gap bound states occur only in d-wave superconductors with nonmagnetic impurities.

Impurity bound state energies vary with doping level and impurity potential.

Magnetic impurities induce sub-gap states in both s-wave and d-wave superconductors.

Abstract

Measurements of the local density of states near impurities can be useful for identifying the superconducting gap structure in alkali doped iron chalcogenide superconductors K_xFe_{2-y}Se_2. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest neighbor d-wave and next-nearest neighbor s-wave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that sub-gap bound states exist only for d-wave superconductors with the positions of these bound states depending rather sensitively on the electron doping level. Further, for such disorder Coulomb interactions can lead to local impurity-induced magnetism in the case of d-wave superconductivity. For magnetic impurities, both s-wave and d-wave superconducting states support sub-gap bound states. The above results can be explained by a simple analytic model that provides a semi-quantitative understanding of the variation of the impurity bound states energies as a function of impurity potential and chemical doping level.